1. FIELD OF THE INVENTION
The present invention concerns a linear displacement measuring apparatus such as a micrometer for use in measurement of a dimension or a shape of an object to be measured and, more in particular, it relates to an electronic linear displacement measuring apparatus for obtaining a moving amount of a spindle as an electric measurement value by a detector.
2. DESCRIPTION OF THE RELATED ART
An existent micrometer for use in measurement of a size or a shape of an object to be measured has, for example, such structure as detecting a moving amount of a spindle movably held to a measuring apparatus main body by an encoder. Such an encoder includes, for example, an optoelectronic type encoder having a main optical lattice disposed to a rotary plate and an index optical lattice disposed to a fixed plate respectively for optically detecting relative rotational displacement between both of the plates, as well as encoders using an electromagnetic, capacitance or contact type displacement detecting device. Measured signals obtained by the encoder are processed by a processing device for measured signals disposed to the inside of a micrometer and displayed in a digitized form on a digital display section or transmitted as transmission signals to an external equipment.
FIG. 17 and FIG. 18 show an existent micrometer. In FIG. 17, the existent micrometer has an anvil 2 fixed at one end of a frame 1 generally in a U-shaped configuration and a spindle 4 held at the other end of the frame 1. A clamp 7 for fixing the spindle 4 to a predetermined position is disposed to the frame 1.
FIG. 18 shows a structure for holding the spindle 4. In FIG. 18, the spindle 4 comprises a spindle main body 4A in contact, at one end thereof, with an object to be measured and a screw shaft 21 secured at one end to the other end of the spindle main body 4A. The spindle main body 4A is supported by way of a shaft bearing cylinder 11 to the frame 1. The screw shaft 21 is screw-coupled at the other end with an inner sleeve 13 having a plurality of recesses 15. An outer sleeve 14 is disposed to the outer circumference of the inner sleeve 13, and the sleeves 13 and 14 are secured by way of a holding ring 12 to the frame 1. The inner sleeve 13 comprises a sleeve main body 13A and a tapered nut 17 screw-coupled with the sleeve main body 13A.
A thimble 3 is secured to the other end of the screw shaft 21, and a rotational operation made to the thimble 3 causes the spindle 4 to move forwardly and backwardly relative to the anvil 2, so that measurement is conducted by means of a main scale 23 and a sub-scale 24 in a state of gripping an object to be measured. A ratchet 22 is disposed for gripping the object to be measured at a constant pressure.
In the existent electronic micrometer, a moving amount of the spindle 4 is converted into electric measured signals by an encoder 41 having a fixed plate 42 fixed to the holding ring 12 and a rotary plate 43 fixed to the spindle 4, the measured signals are processed by a not illustrated processing device having an electric circuit and displayed on a digital display section 5 or transmitted to other signal processing sections.
However, when an existent micrometer is used in such a working circumstance that liquids such as grinding liquids or grinding oils are always scattering around, it results in a problem that the liquid intrudes to the inside of the micrometer, by which a processing device at the inside causes erroneous operation.
Namely, as shown in FIG. 18, liquid intruding paths, for example, paths a1, a2 and b through which the liquid intrudes from the outside to the inside are present in the micrometer.
The path a1 is present at a boundary between the outer sleeve 14 and the thimble 3. The liquid intrudes through the boundary between the outer sleeve 14 and the thimble 3, creeps along a small gap 19 between the outer circumferential surface of the outer sleeve 14 and the inner circumferential surface of the thimble 3 and then reaches the outer circumferential surface of the screw shaft 21 through the recesses 15 of the inner sleeve 13 as shown by the path a2. Since the screw shaft 21 is formed integrally with the spindle main body 4a, the intruding liquid creeps along the screw shaft 21 and reaches the encoder 41 and the processing device attached to the spindle main body 4A.
The path b is present at a boundary between the spindle main body 4A and the shaft bearing cylinder 11. The liquid intrudes from the path b, further, intrudes through a boundary 34 between a cylindrical member 31 disposed in adjacent with the shaft bearing cylinder 11 and the spindle main body 4A to the inside of the micrometer and reaches the encoder 41 or the processing device.
A chamber 25 is formed to the inside of the thimble 3, and the volume of the chamber 25 changes greatly by rotating to axially move the thimble 3. If the volume change occurs abruptly, a difference may be caused between an external atmospheric pressure and a pressure in the chamber 15 and between the pressure in the chamber 25 and a pressure at the inside of the micrometer in which the processing device is contained.
For instance, if the external atmospheric pressure is higher than that in the chamber 25, the liquid at the outside intrudes passing through the path a1 into the chamber 25. If the pressure in the chamber 25 is higher than the pressure in the micrometer, the liquid intruding into the chamber 25 creeps from the path a2 and along the screw shaft 21 and then reaches the encoder 41 and the processing device disposed to the inside. That is, a sort of pumping action is given by the chamber 25.
Therefore, in the existent micrometer, the liquid may some time intrude through the liquid intruding paths a1, a2, b to a detector such as a processing device comprising electric circuits, which makes measurement impossible or causes measuring errors.
It is, accordingly, an object of the present invention to provide a water proof linear displacement measuring apparatus capable of preventing a liquid from intruding into a detector disposed to the inside of a measuring apparatus main body thereby eliminating measurement failure caused by the liquid.